The nature of consciousness remains deeply mysterious and profoundly important, with existential, medical and spiritual implication. We know what it is like to be conscious – to have awareness, a conscious ‘mind’, but who, or what, are ‘we’ who know such things? How is the subjective nature of phenomenal experience – our ‘inner life’ - to be explained in scientific terms? What consciousness actually is, and how it comes about remain unknown. The general assumption in modern science...

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Quantum Mind-Quantum States In The Retina?

Quantum States In The Retina?

Pierre St. Hilaire

Dick Bierman

Stuart Hameroff

Our approach to the problem of consciousness implicates quantum coherent states in the brain. Conventional wisdom in science holds that significant quantum phenomena are impossible at brain temperature due to thermal decoherence. However theory suggests that the brain has evolved methods to avoid decoherence, and in fact uses thermal energy to pump the coherence. Experimental observation of such quantum states has not yet been possible (quantum consciousness models are testable, whereas conventional classical theories of consciousness are, in general, not testable).

Recently physicists Pierre St. Hillaire and Dick Bierman at Starlab devised an experimental system to utilize "photon echo", a technique from quantum optics, to look for evidence of quantum coherent superposition in the human retina (a part of the brain) in awake volunteers. In photon echo a short laser pulse is sent to the system being studied followed by another pulse from the same source. If quantum coherence is occurring in the system a delayed photon "echo" is detected. Because the laser pulses required are of a very low power there is no risk of injury to the retina. Indeed subjects should perceive only a faint flash of light. Lasers are used commonly in ophthalmology; safety of the technique will be assured in the experiments proposed, and possible spinoff effects of retinal holography evaluated.

The idea is that the first pulse when reaching the retina causes rhodopsin molecules (and perhaps others) in the rod and cone cells in the retina to go into a state of quantum coherent superposition. If these quantum coherent states persist for a significant time duration (the "decoherence time") longer than the interval between the two laser pulses, then the second pulse causes some of the atoms to "precess back towards their initial state where the first pulse was encountered and then emit a photon corresponding to the initial quantum transition (hence the name "photon echo")". This photon echo can be detected with sophisticated quantum optical equipment, and would indicate a state of quantum coherent superposition in the retina/brain. Decoherence times for the quantum state in the nanosecond range or higher would be highly significant and supportive of quantum consciousness theory.

The experimental protocol is included here:

INVESTIGATION OF DEPHASING TIMES IN THE HUMAN RHODOPSIN COMPLEX BY PHOTON ECHO EXPERIMENTS

Pierre St. Hilaire Interval Research Corp., Palo Alto, USA Dick J. Bierman University of Amsterdam, The Netherlands StarLab, Brussels, Belgium SUMMARY It has been speculated that structures in the human brain could, at some level, exist in a state of coherent superposition. In this research proposal an experiment is worked out that tries to assess decoherence times of quantum states in the human retina, an extended part of our brain, using the standard photon echo technique from quantum optics. That the brain or visual pathways can exhibit quantum coherence over macroscopic time intervals (say, milliseconds) is almost as hard to swallow for a physicist as is faster than light communication, since at room temperature typical decoherence times are measured in femtoseconds.

The only long-lived macroscopic quantum superposition states of matter demonstrated so far for exist in exotic systems such as rare earth ions embedded in crystal matrices at a few degrees Kelvin, Bose-Einstein condensates at nanokelvin temperatures, superconductors, or ions held in electromagnetic traps. Despite these caveats there is a school of scientists that maintains that conscious awareness indeed depends upon quantum computations being carried on in the brain. The measurement of stimulated photon echoes over intervals of nanoseconds or more would prove without ambiguity that structures in the retina such as the rhodopsin complex exhibits macroscopic quantum superpositions and could represent a fundamental advance in our study of vision and visual awareness. In the proposed photon echo experiments a short laser pulse is initially sent to the the rhodopsin complex in the retina at a frequency corresponding to a quantum transition, followed by another pulse from the same source. If the time interval DT between the two pulses is less than the dephasing time T2 of the rhodopsin complex then a photodetector will detect an optical signal at a time DT after the second pulse: the photon echo The photon echo rapidly diminishes as DT becomes larger than the dephasing time T2 of the rhodopsin complex. Although the experiment is conceptually simple the observation of the stimulated echoes coming from the back of the eye is complicated by the very small number of photons that would have to be measured - thus requiring the use of detectors sensitive at the single photon level, low noise electronics, and careful laboratory techniques. These are described in the section on experimental protocol. The proposed experiment is as speculative as the proposal that quantum coherence might play a role in the brain. Thus some estimate of the probability that we could find anything of that kind is necessary. The specific question then is: Would it be possible for the retinal molecule to exist in a coherent superposition of the cis and trans states within the rhodopsin complex for macroscopic amounts of time? And, more importantly, could a large number of these molecules coexist in the cone and rod cells without interacting with each other (and thus introducing dephasing)? There has been no experiments reported that directly address this question but work on investigating the mechanisms by which rhodopsin isomerizes suggests that the photochemistry occurs from a vibrationally coherent system.

BACKGROUND

There have been a lot of discussions (e.g. Penrose & Hameroff, 1996) about the fact that states of the human brain could at some level exist in a state of coherent superposition. This of course is very hard to demonstrate in the general case and is highly contested. But there is a small subset of the human brain that is directly observable: the retina. An interesting and active area of research in neuroscience focuses on investigating a very small subset of consciousness, visual awareness, through the brain pathways. Various experiments are slowly unraveling how we become aware of visual information with tools such as functional MRI. Maybe we should start to investigate the conservation of quantum coherence through the visual pathways. The human eye surely seems to take great care of conserving coherence all the way to the retina since the image created on the fovea is essentially diffraction limited. For natural light any optical path difference greater than a fraction of a micron between object and image will degrade the image. In the language of QM natural light photons separated by more than a few wavelengths (the so-called coherence length) will become distinguishable and will no longer interfere. Laser light, of course, has a much longer coherence length, which is why it exhibits speckle. What if the coherence state of natural light could somehow be preserved thorough the neural processing? After all, the receptors in the retina are quantum detectors. The classical vision model is that the wavefunction corresponding to the visual information in a scene somehow "collapses" at the first layer of the retina, with all the subsequent processing done classically - in a fashion similar to the case of, say, a video camera linked to a processor. But what if the "collapse" happened much later in the visual processing? Or in the words of J.S. Bell, where does the measurement of a visual field happen? Is it right at the first retinal layer, or does it happen further up in the visual cortex? That the brain or visual pathways can exhibit quantum coherence over macroscopic time intervals (say, milliseconds) is almost as hard to swallow for a physicist as is faster than light communication, since at room temperature typical decoherence times are measured in femtoseconds. The only long-lived macroscopic quantum superposition states of matter demonstrated so far for exist in exotic systems such as rare earth ions embedded in crystal matrices at a few degrees Kelvin, Bose-Einstein condensates at nanokelvin temperatures, superconductors, or ions held in electromagnetic traps. Despite these caveats there is a school of thought that maintains that conscious awareness indeed depends upon quantum computations being carried on in the hot and wet brain. Amazingly enough there are examples of living systems having harnessed coherent quantum superpositions (Hu and Schulten 97). Purple bacteria (which have existed for over a billion years) are able to coherently superpose electronic excitations derived from sunlight to perform photosynthesis (an excellent reference on the subject can be found in the August 1997 issue of Physics Today). And the rod and cone photoreceptors in the retina are quantum detectors sensitive at the photon level. So it is not completely absurd (although highly unorthodox) to suppose that nature could have further harnessed quantum subtleties along the path of evolution for the purpose of information processing. It is difficult, however, to visualize processes preserving coherence over microseconds or milliseconds as opposed to the picosecond scale measured in purple bacteria. The rhodopsin complex is responsible for the initial detection of a photon in the retina (Stryer 87). Rhodopsin has two components: 11-cis retinal and opsin. Opsin is a complex protein that has the form of seven helical structures. 11-cis retinal is a much simpler molecule that is cradled in the opsin protein. The absorption of a photon by retinal converts the cis-retinal to the all-trans form and activates a complex chain of biochemical events that culminates in the excitation of the optic nerve. Would it be possible for the retinal molecule to exist in a coherent superposition of the cis and trans states within the rhodopsin complex for macroscopic amounts of time? And, more importantly, could a large number of these molecules coexist in the cone and rod cells without interacting with each other (and thus introducing dephasing)? A bibliographical search did not find any evidence of experiments aimed at demonstrating this hypothesis. There has been some work, however, in investigating the mechanisms by which rhodopsin isomerizes. Interestingly, these investigations suggest that the photochemistry occurs from a vibrationally coherent system (Schoenlein 91). If macroscopic quantum coherence is maintained in the human retina it should be possible to measure it with the conventional tools of quantum spectroscopy. In particular, in vivo time resolved laser spectroscopic measurements of the retina should exhibit the range of effects found only when performing spectroscopic measurements of isolated atomic systems such as rare earth ions in crystalline matrices. Such effects include spectral hole burning, photon echo, and observation of very narrow spectral bands (Mandel and Wolf, 1995). Such experiments (if positive) would bring an unambiguous proof of macroscopic quantum coherence in the retina and would have important technological consequences as well. To our knowledge such in vivo measurements have never been done. The rest of this article describes a proposal for such an experiment, based on the photon echo phenomenon.

PHOTON ECHO

Photon echoes are a standard tool of nonlinear spectroscopy for determining the dephasing time T2 of an atomic population (Mandel and Wolf, 1995). In photon echo experiments a short laser pulse is initially sent to the population at a frequency corresponding to a quantum transition, followed by another pulse from the same source. If the time interval DT between the two pulses is less than the dephasing time T2 of the population then a photodetector will detect an optical signal at a time DT after the second pulse: the photon echo. In essence, the second pulse causes some of the atoms to precess back towards their initial state where the first pulse was encountered and then emit a photon corresponding to the initial quantum transition (hence the name "photon echo"). The photon echo rapidly diminishes as DT becomes larger than the dephasing time T2 of the atom population. So far the longest dephasing times (up to a few hundred microseconds) encountered by the technique in solids have been found in rare earth ion dopants in crystalline matrices at a few degrees K. The dephasing time rapidly diminish as the temperature of these systems is raised due to phonon interactions. We propose to perform the same type of experiments to measure the decoherence time of the rhodopsin complex in the retina. Because the laser pulses required are of a very low power there is no risk of injury to the retina - indeed an observer would only perceive a faint flash of light. Although the experiment is conceptually simple the observation of the stimulated echoes coming from the back of the eye is complicated by the very small number of photons that would have to be measured - thus requiring the use of detectors sensitive at the single photon level, low noise electronics, and careful laboratory techniques. The equipment and techniques required are similar to those used in experimental investigations of the quantum mechanics of entangled systems, for example. We plan to use Bragg cells as photon gating devices in our experiments. The laser needs to be carefully stabilized in frequency with an external etalon but needs to be of low power (at the milliwatt level). All of the required equipment is available off the shelf. The measurement of stimulated photon echoes over intervals of nanoseconds or more would prove without ambiguity that the rhodopsin complex exhibits macroscopic quantum superpositions and could represent a fundamental advance in our study of vision and visual awareness.

EXPERIMENTAL PROTOCOL

A simplified schematic of the optical setup of our experiment is presented in Figure 1. The optical layout is very similar to the one used in conventional photon-echo measurements. The light source is a frequency-stabilized continuous wave (CW) laser of low power (only a few milliwatts are required). Ideally the laser should be tunable over part of the visible spectrum (to repeat the experiment at different wavelengths), but a fixed frequency laser will be adequate for initial experiments. We are considering using either a frequency-stabilized Helium-Neon (633 nm, red) or doubled ND:YAG (532 nm, green) laser because of their availability and moderate cost. The laser beam then goes through an acousto-optic modulator (AOM 1) which acts as fast shutter (with rise time of tens of nanoseconds). The beam is then expanded by lenses L1 and L2 and is shined onto the viewer's eye, where it is focused onto the retina. A diffuser lightly spreads the beam so that a small patch of the retina is illuminated as opposed to a single spot. The diffuser acts to limit the field intensity at focus and ensure that many photoreceptors are illuminated during each pulse. Light coming from the retina is diverted by the beamsplitter BS to the combination of lenses L3, L4, and a pinhole that acts as a spatial filter. The spatial filter augments the signal to noise ratio by blocking photons coming from outside the illuminated retinal patch. The light is then gated by AOM2 and focused onto the detector. After proper amplification the output of the detector is displayed on a high-speed sampling oscilloscope (or can be digitally sampled and further processed on a computer). A simple timing generator controls the AOM's and triggers the data acquisition. The apparatus operates as such: An optical pulse is initially generated by AOM1, followed after an interval DT by a second short pulse having twice the amplitude of the first (each pulse lasts on the order of 10 nanoseconds). AOM2 is turned off during the initial two pulses to prevent detector saturation. After those two pulses AOM2 is turned on and the signal recorded. The stimulated photon echo should be detected as a weak signal arriving at the detector at an interval of DT after the second pulse. The photon echo intensity will diminish rapidly as the interval DT approaches the dephasing time T2 of the system under study (e.g. the retina). We can thus determine the dephasing time by varying DT in a number of successive experiments. The experiment poses no danger of retinal damage because of the short duration of the pulses and low intensity of the laser. Indeed the subject would only perceive a weak flash as the experiment proceeds. Figure 1. Optical layout of experiment Although conceptually simple, the experiment requires great care in its implementation because of the small number of photons present in the detected echo. This is because the initial stimulated echo will propagate along the same direction as the original laser beam (i.e. towards the back of the retina). Only the photons that are subsequently backscattered by the retina will be detected, which is likely to represent a small fraction. Since the echo is very small any stray photons hitting the detector might generate false counts. A very low noise detector and amplifier is also required. This problem, however, can be overcome by careful experimental techniques that have become the mainstay of quantum optics measurements. Suitable detectors capable of single photon detection are commercially available from Hamamatsu. It has been argued that a much easier experiment could be performed on purified rhodopsin. However, at room temperature, rhodopsin probably exhibits very short decoherence times (from femto to picoseconds) because of the dephasing effects introduced by phonons (Remember that so far macroscopic dephasing times in solids have been measured only in systems cooled to a few degrees K). Our thesis is that somehow a process is active in the retina that extends the decoherence time by orders of magnitude. BUDGET All the components are available off the shelf and none is especially exotic. However, the bill of material will probably exceed the $20 000 limit of this grant proposal unless we can benefit from existing facilities having some of the required equipment. Any optical laboratory should have a suitable set of lenses, mounting hardware, and data acquisition hardware. Single frequency lasers are commonly used in labs performing interferometric experiments or holography. The Bragg cells and associated drivers cost about $1000 each, and the detectors and low noise amplifiers can also be procured for a few thousand dollars. The ideal solution would be to work within an optical laboratory having an ongoing quantum optics experimental program, since all of the necessary components are used in such experiments. At present we consider two options. The first option is to situate the research at the FOM institute for Atomic and Molecular physics (www.amolf.nl) where Dick Bierman did his PhD work. Much of the needed equipment would be available and could be rented rather than purchased. However this is dependent on the use that other internal projects make of the needed equipment. We intend to explore the possibilities there when the status of this proposal becomes clearer. A second option is to situate the research at StarLab (www.starlab.org). This institute might be willing to invest some additional funding also for the labor involved. TIMETABLE It should be possible to obtain initial results within a few months of the start of the program given an adequate laboratory and equipment. The entire experiment should not last over 6 months.